Combinations of inhibitors have been heretofore proposed which are effective in slowing the rate of crack propagation in high strength steel. "Inhibition of Crack Propagation of High Strength Steels through Single and Mulifunctional Inhibitors" AFML-TR-76-120, August 1976 describes how crack growth in high strength, low alloy steels may be retarded when maintained in a aqueous solution by the presence of oxidizing inhibitors such as chromate or by the presence of nitrite ions. It also demonstrates that a combination of borax and nitrite are more effective than single inhibitors in controlling crack propagation in high strength steels provided these are maintained in liquid. Experimental results showed this system to be equally effective in the presence of sodium chloride and in both sustained load stress-corrosion and cyclic load corrosion-fatigue conditions.
The literature also contains data on the use of inhibitors to slow crack growth in aluminum alloys. Again, an aqueous solution is essential to this operation. "Mechanisms of Corrosion Fatigue Crack Propagation in Al-Zn-Mg Alloys" MIT, Department of Metallurgy and Materials Science, February 1972 discloses that the addition of nitrite ions drastically reduces the corrosion fatigue crack growth rates of peak aged 7075-T6 aluminum tested in sodium chloride solution. The nitrite ions displace the chloride ions from the crack tip and change the fracture morphology from brittle to ductile. In "Material Evaluation: Part II-Development of Corrosion Inhibitors", AFML-TR-79-4127, September 1979, retardation of crack growth in high strength aluminum alloys exposed to distilled water and to 0.1 Molar sodium chloride solutions is effected through the use of borax-nitrite and piperazine inhibitors.
In summary, the prior art disclosures on the use of inhibitors to control crack growth rate are predicated upon a continuous exposure of the crack path to solutions which contain the inhibitors. In the airplane application there is no liquid to carry the inhibitor to the crack tip and surfaces. At best, therefore, the prior art practices have been to periodically wash or rinse the aircraft with solutions containing the various inhibitors in an effort primarily to remove all salt residue which are known to accelerate crack growth.